1. WFCAM Photometric Calibration
Simon Hodgkin ++
Main points:
Long term strategy
Actual plan
Nightly observing strategy
Dominant effects to monitor/measure:
Spatial systematics
Colour effects
Chip-to-chip variations
Extinction
Time dependence ?
Airmass dependence
Choice of fields
Primary standards
Secondary standards
Mechanics
Funny filters
11/27/2018 Simon Hodgkin CASU

2. Overview
Conversion from WFCAM counts to Vega magnitudes at airmass unity in the MKO-NIR system
The goal is to achieve this to 2% accuracy (UKIDSS)
Looking to produce two papers:
(i) modelling of the end-end system
(ii) empirical measurment of the WFCAM photometric system
J = ZPJ – Jinst – kJ (– 1)
strictly kJ = kJ’ + kJ’’(J – K)
11/27/2018 Simon Hodgkin CASU

3. Flies in the ointment spatial systematics extinction colour dependence
scattered light
flatfield errors
variable pixel scale
geometrical – vignetting/secondary reflectivity
extinction colour dependence
extinction time dependence
chip-to-chip gain dependence
chip-to-chip QE colour effects
filter colour terms (time variable?)
11/27/2018 Simon Hodgkin CASU

4. Scattered Light in WFI
WFI V-band observation of a Landolt standard field. Right hand panel shows the results of applying a quadratic correction term:
mag = a ( 2 + 2 )
11/27/2018 Simon Hodgkin CASU

5. WFCAM characterisation plan
detector noise properties
microstepping test
sky emission
fringing
sensitivity
background limit
cosmic rays
persistance
flatfield
scattered light: meso-step field, effect of bright stars
area calibration: chip-chip/channel-channel
astrometry
guiding
photometry: set up secondary standard fields
11/27/2018 Simon Hodgkin CASU

6. Primary Standards
JAC are observing standards with the Mauna Kea consortium filter set in UFTI (Simons and Tokunaga 2002, Tokunaga et al. 2002)
WFCAM uses the same JHK filter system
>100 UKIRT standards with (JHK)MKO-NIR which will not saturate a 1s WFCAM exposure (about 50 for a 5s exp)
Preliminary results show persistence effects are small (2e-4 after 20s)
The UKIRT standards therefore make excellent primary standards for WFCAM
Y,Z and narrow-band filters require extra work
11/27/2018 Simon Hodgkin CASU

7. Secondary Standards By defining standard fields we: Choose fields:
Beat down the noise
Allow for variables
Can measure spatial systematics
Can measure colour-terms (can change)
Calibrate 4 detectors simultaneously
Choose fields:
Spaced every 2 hours in RA
Equatorial (good for VISTA)
=+20 degrees (X=1.0)
11/27/2018 Simon Hodgkin CASU

8. Possible standard fields
Around UKIRT standards
100s of stars, measured simul. with primary std
Mostly red
Near Galactic Plane (5h45+18, 7h15+00, 17h50+00, 20h30+18)
1000s of stars, avoid worst crowding
Globular clusters (NGC5053, M3)
Horizontal branch for blue stars
Small, dense cores
Open clusters (Pleiades, Praesepe)
Numerous, large areal coverage
Not many stars
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9. NGC5053: 2MASS J
8 arcminutes
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10. NGC5053 source counts
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11. standard fields for WFCAM
fields centred on primary standards from Persson et al. (1998) and UKIRT FS (Harwarden et al. 2001, Leggett et al. 2003)
equatorial strip + overhead (=1) =+20: MKO, =-25: Paranal
used 2MASS catalogue to select fields with >100 stars per chip (to J=16)
64 equatorial, 13 northern, 12 southern
further refinement required
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12. WFCAM/VISTA Standard fields
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13. Pre-WFCAM observations?
How photometrically stable is MKO, e.g. with humidity?
How does extinction vary with time on a wet vs dry night?
Enough data to investigate this?
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14. UFTI standards data span Jan 2000 to Oct 2003
only selected nights with >=4 stds in each of J,H,K
delta AM >0.5
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15. Humidity
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16. How many standards?
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17. Nightly Calibration
We must measure enough standards every night to ensure that the photometricity of a (fraction of a) night can be derived from the data.
Overheads
E.g. ( (3x5s) +20s ) x 5 filters + slew + acq
5s S/N=100 J=15 (5 min total)
30s S/N=100 J=16 (11min total)
Frequency
Hourly - this will not be under the observer’s control
Extinction
Do we measure it – nightly/hourly/at all ?
11/27/2018 Simon Hodgkin CASU